1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.X simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,O27,477, 4,130,095, 4,235,204). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor in addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, for example, an air-fuel ratio feedback control parameter such as a rich skip amount RSR and/or a lean skip amount RSL is calculated in accordance with the output of the downstream-side O.sub.2 sensor, and an air-fuel ratio correction amount FAF is calculated in accordance with the output V.sub.1 of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter as illustrated in FIGS. 2A and 2B (see: U.S. Ser. Nos. 831,566 and 848,580). In this case, the air-fuel ratio feedback control parameter is stored in a backup random access memory (RAM). Therefore, when the downstream-side O.sub.2 sensor is brought to a non-activation state or the like to stop the calculation of the air-fuel ratio feedback control parameter by the downstream-side O.sub.2 sensor, the air-fuel ratio correction amount FAF is calculated in accordance with the output of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter which was calculated in an activation state of the downstream-side O.sub.2 sensor (i.e., an air-fuel ratio feedback control mode for the downstream-side O.sub.2 sensor) and was stored in the backup RAM.
In the above-mentioned double O.sub.2 sensor system, however, when the control is transferred from an air-fuel ratio feedback control mode for the downstream-side O.sub.2 sensor to an open-loop control mode for the downstream-side O.sub.2 sensor, the air-fuel ratio feedback control parameter may be so large or small that an air-fuel ratio feedback control by the upstream-side O.sub.2 sensor using the air-fuel ratio feedback control parameter invites overcorrection of the air-fuel ratio. That is, in an air-fuel ratio feedback mode for the upstream-side O.sub.2 sensor, when the catalyst converter is not completely activated or when the upstream-side O.sub.2 sensor is not completely activated, the air-fuel ratio may be made overrich or overleaned, thus increasing the HC and CO emissions or the NO.sub.X emission. Also, in an air-fuel ratio feedback control mode for the upstream-side O.sub.2 sensor, when the engine is in an idling state, the air-fuel ratio may be greatly fluctuated, thus reducing the drivability characteristics. Note that the idling state is usually one condition of the air-fuel ratio feedback control mode for the downstream-side O.sub.2 sensor, but is not a condition of the air-fuel ratio feedback control mode for the upstream-side O.sub.2 sensor.
Also, in the air-fuel ratio feedback control mode for the upstream-side O.sub.2 sensor and in the open-loop control mode for the downstream-side O.sub.2 sensor, it is possible to control the actual air-fuel ratio in accordance with the output of the upstream-side O.sub.2 sensor and an air-fuel ratio feedback control parameter which is a fixed value. In this case, however, the air-fuel ratio feedback control parameter calculated in the air-fuel ratio feedback mode does not reflect the control of the air-fuel ratio in the open-loop control mode for the downstream-side O.sub.2 sensor at all, and accordingly, it is impossible to obtain an optimum air-fuel ratio such as the stoichiometric air-fuel ratio in the open-loop control mode.